Secondary cooling systems: rules for glycol, brine, CO2 and chilled water

What an indirect system is

EN 378-1 defines a secondary cooling or heating system as one that uses a fluid to move heat from the product or space to the refrigerating system without compressing or expanding that fluid, and the standard applies to such systems too ^[3]. Chilled water, glycol or salt brine is piped to the coolers. CO2 can also be used as a volatile secondary: it is pumped as a liquid, partly evaporates in the coolers and is condensed by the primary circuit, at much higher pressures than a glycol loop.

The reasons are a small refrigerant charge in one place, and keeping flammable (A2L, A3) or toxic (B2L, ammonia) refrigerants away from occupied spaces. The price is an extra temperature difference and pump energy.

How EN 378-1 classifies systems

EN 378-1 classifies refrigerating systems in clause 5.4. It separates direct releasable systems, where leaking refrigerant can reach the occupied space, from indirect systems, and clause 5.5 gives examples of both ^[3]. The system class is used with the location class and access category when the charge limit is set; see the article on EN 378 and ISO 5149.

EU F-gas Regulation (EU) 2024/573

The regulation covers fluorinated greenhouse gases. Water, glycol, brines and CO2 are not among them, so the duties follow the refrigerant circuit ^[1]:

• Leak checks from 5 tonnes CO2e of Annex I gases (Article 5(1)), at least every 12, 6 or 3 months from 0, 50 and 500 t CO2e, or every 24, 12 or 6 months with a leakage detection system (Article 5(6)). Tonnes CO2e are the charge in tonnes multiplied by the GWP (Article 3) ^[1].
• Leakage detection system from 500 t CO2e in stationary refrigeration, air conditioning and heat pump equipment (Article 6(1)) ^[1].
• Certificates (Article 10) for installation, maintenance or servicing, repair, decommissioning and leak checks. Maintenance or servicing means work that opens circuits containing fluorinated gases, adds gas or repairs leaks (Article 3(18)). Work only on pumps, strainers or glycol does not open the refrigerant circuit ^[1].

Where the regulation names indirect systems

A chiller is a single system whose main function is to cool a heat transfer fluid, and the regulation names water, glycol, brine and CO2 as examples (Article 3(44)) ^[1]. The unit serving the secondary circuit is therefore usually a chiller, covered by Annex IV point 7:

Points (b) to (d) do not apply where the F-gas is needed to meet safety requirements at the site. The general ban on other refrigeration equipment at GWP 150 or more from 1 January 2030 (point 5(c)) excludes chillers ^[1].

Multipack and cascade. Commercial multipack systems of 40 kW or more with F-gases of GWP 150 or more are banned from 1 January 2022, except in the primary circuit of cascade systems, where GWP below 1,500 is allowed (point 6) ^[1]. Here the secondary circuit is a medium-temperature refrigerant circuit whose condenser heat the primary circuit absorbs, not a brine loop (Article 3(35)) ^[1].

Service bans. Since 1 January 2025, gases with GWP 2,500 or more may not be used to service refrigeration equipment, except reclaimed or recycled gas until 1 January 2030 (Article 13(3)). The ban on GWP 750 or more from 1 January 2032 excludes chillers (Article 13(5)) ^[1].

Switzerland: ChemRRV Annex 2.10

Switzerland has its own rules in the Chemikalien-Risikoreduktions-Verordnung (ChemRRV, SR 814.81), Annex 2.10. They concern refrigerants that are stable in air, such as HFCs. The version in force from 1 January 2027 names the secondary circuit (Kälteträgerkreislauf) ^[4]:

• Commercial and industrial food refrigeration that is self-contained or has a secondary circuit may not be placed on the market with a refrigerant of GWP 150 or more (section 2.1(3)(b)(4)) ^[4].
• For building air conditioning and industrial process cooling, the ban at GWP 150 or more targets direct-expansion systems (section 2.1(3)(a)(5) and (c)(5)) ^[4].
• Refilling with GWP 750 or more is banned in food and industrial refrigeration and ice rinks, but virgin refrigerant remains allowed in systems with a secondary circuit (sections 3.3.1(2) and 3.3.2(2)(b)). For other systems the ban on virgin refrigerant starts after 31 December 2031 (section 7(5)) ^[4].

Pressure equipment: the PED on the secondary side

Directive 2014/68/EU treats a fluid as a liquid when its vapour pressure at the maximum allowable temperature TS is at most 0.5 bar above atmospheric. Water, glycol and brines normally qualify. Group 1 covers flammable, toxic and other hazardous fluids listed in Article 13(1)(a), and fluids with TS above their flashpoint; all others are group 2 ^[2]. Article 4(1) sets these limits:

A glycol loop with PS 10 bar or less is therefore outside Article 4(1) whatever its size. Such equipment follows sound engineering practice, comes with instructions and carries no CE marking under the directive (Article 4(3)) ^[2]. CO2 as a secondary is group 2 but follows the gas limits: at PS 40 bar a DN 40 line gives 1,600 bar and is covered ^[2]. An evaporator or plate heat exchanger with several chambers is classified by the chamber with the highest category (Article 13(2)) ^[2].

Glycol, brines and water protection

Glycols and brines can harm water and soil. National rules on substances hazardous to water may apply; ask the local environmental authority.

Practical checklist

• Count CO2e for the refrigerant circuit only ^[1].
• Check whether the cooling unit is a chiller (Article 3(44)) and use Annex IV point 7 ^[1].
• Opening the refrigerant circuit needs an F-gas certificate; work on the brine side alone does not ^[1].
• Check the system class against EN 378-1 clauses 5.4 and 5.5 ^[3].
• Set PS on the brine side and note whether it exceeds 10 bar; use the gas limits for CO2 ^[2].
• In Switzerland, check the secondary-circuit rules in ChemRRV Annex 2.10 ^[4].

Size the pipes, pumps and pressure drop of a water or glycol circuit with the Heating & cooling (hydronic) calculator.